A conventional subsea power transmission and distribution system is typically an ac system with an onshore step-up transformer, an ac transmission cable, one or more subsea step-down transformers, circuit breakers and frequency converters for powering subsea electrical loads such as compressors and pumps. Such conventional subsea power transmission and distribution systems work well for relatively short offshore distances (e.g. 10-20 km) and where the total power rating is within certain limits.
For longer offshore distances then the ac transmission cable will generate a considerably large reactive current due to its high capacitance. For example, a cross-linked polyethylene cable carrying 132 kV can generate a reactive current up to 1000 kVAR/km. This significantly reduces the active power transfer capability of the ac transmission cable as the offshore distance is increased to 100 km or more. A shunt reactor or some form of static voltage compensator is then needed for reactive power compensation and voltage stability control.
When ac transmission cables are no longer appropriate then it is known to use high voltage direct current (HVDC) transmission for offshore applications. The dc transmission cables are less expensive because they can be operated safely at higher current for a given amount of insulation and copper. A dc transmission cable can therefore transmit more power for a given cable expenditure. The losses in the dc transmission cable are also lower because of the lack of charging currents in the main conductor and the induced currents in the shielding. There is also no resonance between the dc transmission cable and the conventional ac equipment that is associated with the ac network or power grid.
An example of a commercially available HVDC power transmission system is the HVDC LIGHT system that is supplied by ABB Ltd of Zurich, Switzerland and which has been used to provide dc power from an onshore station at Kollsnes in Norway to the Troll A oil and gas platform. It uses VSC converters with a pulse width modulation (PWM) strategy to provide decoupled active and reactive power controls. However, high switching losses for high power applications means that the switching frequency of the VSC converters is limited. The HVDC LIGHT system also requires the use of physically large and costly ac filters and a line-frequency transformer. The physical size of the offshore components can be a significant disadvantage in situations where there are practical restrictions on the size of a converter station.